What are the sfarim that speak about SHOVAVIM?
Do people still fast on Mondays / Thursdays?
What are other minhagim that people have during these special hol holy days?
The Shelah Hakadash on the Torah in Parashas Shemos discusses it based on the pasuk shuvu banim shavovim consisting of parashiyos from beginning letters of Shemos to Mishpatim. In a leap year we add two extra parashiyos T’T or Tat,Trumah and Tetzaveh. T’T comprises all that teshuva should be performed for. Teshuvah Tefila, Talmud Torah and Tat meaning to give, Tzedakah.
He explains that in a leap year extra teshuva is required for the moon being diminished in size requiring an extra month around every three years in 19 years (1,6,8,11,14,17,19) to catch up 354 days moon’s year with 365 days sun’s year.
The above should be years (3,6,8,11,14,17,19).
Only Thursday, not Monday
To those that fast, they are mekabel ta’anis the day before and if there is a minyan, they act as on a regular ta’anis tzibbur. Selichos, avinu malkeinu, va’yechal, aneinu at mincha, etc…
Fasting behav requires three fasts but this would require 6 or 8 fasts. Currently with COVID not very likely, so if you can redeem it.